Radio frequency tuners for selecting a desired channel for reception within a radio frequency band and converting the channel to a standard intermediate frequency output signal are well known. Such tuners have many applications including receiving signals from cable distribution networks, terrestrial aerials and satellite aerials and are generally of the superheterodyne type. One known type of tuner of the single conversion type performs conversion from the selected channel to the output intermediate frequency in a single frequency conversion step. Another known type of tuner is of the double conversion type and converts the selected channel to a much higher intermediate frequency in a first conversion stage and then to a much lower standard intermediate frequency in a second conversion stage. Tuners having more than two conversion stages are also known.
In a typical known type of multiple conversion tuner such as a double conversion tuner, all of the frequency-selective filtering is performed by one or more fixed filters. Such filters are generally of the bandpass type with fixed centre frequencies and fixed passbands or bandwidths. The input of the first conversion stage receives a broadband signal with little or no band-limit filtering. The first frequency changer performs an up-conversion to a first intermediate frequency which is higher than the input band. The first intermediate frequency is filtered by an intermediate frequency filter with the desired channel centred on the first intermediate frequency.
The filtered signal from the first intermediate frequency filter generally comprises a small number of individual channels which are supplied to a second frequency changer which performs down-conversion to a much lower standard intermediate frequency. The second frequency changer is connected to a second intermediate frequency filter and the conversion is performed such that the selected channel is centred on the second intermediate frequency with the second filter passing the selected channel and substantially rejecting adjacent channels.
The first and second frequency changers each have a local oscillator controlled by a phase locked loop frequency synthesiser. Each channel is selected by appropriately selecting the frequencies of the local oscillators of the first and second frequency changers with the filtering being performed by non-tracking filters of substantially fixed characteristics.
In order to reduce the cost of such tuners, it would be desirable to implement the whole of a tuner in a single monolythic integrated circuit. Although it is possible to embody fixed frequency-selective filters, for example for the first intermediate frequency filter, in such an integrated circuit, it is impossible at present to implement such filters with accurate and repeatable passband centre frequency characteristics so that “single chip” implementations of such tuner architectures have not been possible.
In the case of known tuners provided with discrete or off-chip filters, the filters constitute a substantial percentage of the cost. Thus, multiple conversion architectures have not been economically viable for many low cost systems, such as terrestrial analogue receivers and cable modems which are extremely cost-sensitive.
GB 2 344 710 discloses a technique for aligning a voltage controlled oscillator of a phase locked loop demodulator.
GB 2 244 877 discloses a technique for periodically recalibrating an over-controlled crystal oscillator in an SSB receiver. The receiver is tuned to a frequency standard and the oven temperature is varied so as to vary the frequency of the crystal oscillator. The frequency deviation at the output of the receiver demodulator is monitored so as to establish the crystal oscillator frequency as a function of temperature. From this characteristic, the optimum oven temperature is determined and set. The oscillator frequency is then reset for zero deviation at the detector output.
GB 2 205 460 discloses a technique for the periodic recalibration of a voltage controlled oscillator, for example in a frequency changer of a pager. A temperature compensation function is stored in the form of a look-up table. The output frequency is periodically checked and, if incorrect, a readjustment is performed and the new point value is substituted in the look-up table for future use.
U.S. Pat. Nos. 4,956,711 and 4,736,456 are concerned with providing an automatic alignment or realignment technique for radio frequency filtering ahead of the frequency changer in a television receiver. A signal is injected into the RF stage with a frequency midway between the sound and vision carriers. The IF strip contains a synchronous demodulator whose local oscillator is re-tuned so as to centre the frequency-converted injected signal on the IF pass band. The AGC time constant is reduced so that the AGC circuit can be used to measure the amplitude of the injected signal. The RF filtering is then adjusted so as to maximise the detected injected signal. The technique disclosed in U.S. Pat. No. 4,736,456 makes use of an actual received sound carrier to adjust one of the RF filter sections.